This invention relates to on-line fiber orientation sensors and more particularly to the control of fiber orientation of a paper web using multiple measurements emanating from such sensors.
Fiber orientation in papermaking refers to the preferential orientation of the individual fibers on the web. Because of flow patterns in the headbox and the jet impingement on the wire, fibers have a tendency to align in the machine direction (MD) versus other directions in the web. For example, it is very easy to tear a square coupon from your daily newspaper in one direction, usually vertical, but not that easy to tear the coupon in the other direction as the newsprint sheet has more fibers aligned in the MD which is typically the vertical direction in a printed newspaper.
If all of the fibers in the web were perfectly distributed, the paper sheet would have the same properties in all directions. This is called an isotropic sheet and its fiber distribution can be plotted on a polar graph in the form of a circle. A fiber ratio, which is the ratio of maximum to minimum fiber distribution 90xc2x0 apart, can be defined for a paper sheet. An isotropic sheet has a fiber ratio of one.
If there are more fibers in one direction than in other directions the fibers are distributed non-uniformly and the sheet is anisotropic. As shown in FIG. 6, the anisotropic fiber distribution can be plotted on a polar graph as a symmetrical ellipse-like geometric figure 72. An anisotropic sheet has a fiber ratio greater than one and with higher fiber ratios the polar distribution tends to be in the shape of a figure eight. The fiber ratio (anisotropy) is defined as the ratio of maximum to minimum distribution, 90xc2x0 apart. The fiber angle xcex1 is defined as the angle of the major axis 76 of the ellipse 72 to the machine direction 74. FIG. 6 illustrates the definitions of FO ratio (the ratio of max 80 to min 82) and FO angle of fiber distribution in a paper sheet.
A fiber orientation (FO) sensor provides the measurement of the fiber angle and the fiber ratio of a paper sheet in both the temporal or machine direction (MD) and also the spatial or cross-machine direction (CD) when it measures across the moving paper web. Each FO scanning sensor can simultaneously produce four profiles of FO measurement. They are the FO angle profile and the FO ratio profile for the topside and the bottom side of the sheet. The typical FO profiles are illustrated in (a) [topside FO angle], (b) [topside FO ratio], (c) [bottom side FO angle] and (d) [bottom side FO ratio] of FIG. 7. These measurements are directly or indirectly linked to other sheet properties like strength and/or sheet curl and twist. One example of a FO sensor is described in U.S. Pat. No. 5,640,244, which issued on Jun. 17, 1997 the disclosure of which is hereby incorporated herein by reference. That patent is assigned to a predecessor in interest to the assignee of the present invention.
In many papermaking processes the flow pattern in the headbox and on the wire makes the fiber distribution on the topside of the web, known as the felt side, different from the fiber distribution on the bottom side of the web, known as the wire side. It is typical to have a larger value of fiber ratio on the wire side than on the felt side. The FO sensor can be designed to separately measure topside and bottom side fiber orientation distribution of the sheet. The bottom side fiber angle is defined looking from the topside to the bottom side.
Some papermaking processes incorporate multiple headboxes with each headbox contributing to a single layer or ply of the final paper sheet. In such a multi-ply configuration, the top and bottom fiber orientation measurements are influenced by completely different headboxes. In single headbox paper machines, the top and bottom fiber orientation measurements are influenced by the same headbox.
Adjusting headbox jet-to-wire speed difference (Vjw=Vjxe2x88x92Vw) can change the FO distribution in a paper sheet. FIG. 8 shows how the FO measurements of one side of a sheet are affected by changing the jet-to-wire speed difference of one headbox. In FIGS. 8(a) and 8(b), both FO angle and ratio profiles are plotted as the contour map for a time period of approximately 100 minutes. The corresponding trend of jet-to-wire speed difference is also shown in FIG. 8(c).
It is advantageous to produce paper products with desired sheet strength and/or curl and twist specifications. The measurements provided by the on-line FO sensor may be used as the inputs to a controller to provide a closed-loop FO feedback control. The ultimate objective of FO control is to adjust the process so that the process can produce sheets with specific paper properties.
U.S. Pat. Nos. 5,022,965; 5,827,399 and 5,843,281 describe various methods and apparatus for controlling fiber orientation but do not disclose or even suggest the controller of the present invention.
The controller of the present invention provides a first step of closed-loop FO control, also known as base level FO control (BFOC). In this first step of FO control instead of achieving desired sheet properties such as strength and/or curl and twist, the BFOC attempts to achieve one or multiple indices that are derived from on-line FO measurements. These indices can for example be an average of FO profile, a tilt index of the measured profile, a concavity index of the measured profile, a signature index of a FO profile, or their combination. A generalized algorithm is provided to transform the raw fiber ratio and fiber angle profiles into these indices, which can be used for control of sheet-forming processes. These indices accentuate the temporal and/or spatial properties of the FO measurements of a manufacturing sheet.
An operator can use the controller of the present invention to produce paper products at different fiber ratio and/or fiber angle settings. Ultimately, with accumulation of experience and knowledge, the repeatable correlation between sheet properties and FO specifications will be established and a supervisory FO control will be created on top of this level of FO controller.
The current invention includes signal-processing methods to transform the FO profile measurements into meaningful indices and controllers to derive effective FO control actions. Originating from the FO sensors are top and bottom fiber angle and fiber ratio raw measurements. These raw measurements comprise vectors of multiple data box values representing FO properties at different cross directional points on the paper sheet. There are four such vectors made available at every completion of scanning at the edge of sheet and they represent profiles of top fiber angle, top fiber ratio, bottom fiber angle and bottom fiber ratio. As was described above, FIG. 7 illustrates typical four FO profiles obtained from a scanning FO sensor. In a generalized sense, these profiles can be treated as continuous functions of CD position. Each of these profiles is subject to filtering in the cross-direction using accepted windowing filters such as Hanning, Blackman, and wavelets. Such filtering techniques allow for capturing the dominant variation of the individual profile shapes.
In order to establish an effective indication of the impact from process adjustments, each FO profile vector can be transformed to a scalar value, which can serve as an index for the associated measurement. A scale index is obtained by convolving a measured FO profile function with a reference function. FIG. 9 shows several examples of reference functions such as the unit step function of FIG. 9(a) and the asymmetrical step function of FIG. 9(b). Here are four example indices which are used herein for the purposes of illustration and not limitation. The first index is an average of all the individual data points that are part of the profile. The second index is termed the tilting index of the profile. The third index reflects the concavity of the profile. The fourth index is called the signature index of the profile. Any combination of these indices can be used as an index of the FO measurement to provide a measured value for a controller.
The controller which is part of the current invention adjusts a manipulated variable to achieve a desired FO target associated with the inferred FO index and is named the base level fiber orientation control (BFOC). This controller is implemented as a single-stage fuzzy controller, a multi-stage fuzzy controller, or the combination of fuzzy controllers with non-fuzzy logic controllers. Using rule-based fuzzy techniques allows the controller to adapt to changing process conditions including a change in the sign of the process gain and non-linearity in the process gain. Each BFOC uses one or multiple FO inferred indices and targets to be achieved as the main inputs. The output from the BFOC is the incremental adjustments to manipulated variables such as headbox jet-to-wire speed difference, slice opening, slice screw settings, edge flows, and/or recirculation flows. Papermakers can attain different control objectives by utilizing the different combinations of derived FO indices.